This invention relates to a device for deriving a synchronizing signal from an electric signal alternately comprising blocks of information and the synchronizing signal, including
a synchronizing signal detector having an input for receiving the electric signal and an output for supplying a detection signal,
a flywheel counter having an input coupled to the output of the synchronizing signal detector, a first output for supplying the synchronizing signal and a second output for supplying a control signal, and
a signal combination unit having a first and a second input coupled to the output of the synchronizing signal detector and to the second output of the flywheel counter, respectively, and an output coupled to the input of the flywheel counter for restarting the flywheel counter under the influence of the signals presented to its inputs.
A device of this type is known from U.S. Pat. No. 3,581,010. The known device has for its object to synchronize the flywheel counter by means of the detection signal which is not directly used as a synchroniing signal, which counter supplies electric signal pulses with a period corresponding to the period of the synchronizing signal. These pulses are used as a synchronizing signal. A missing synchronizing pulse in the electric input signal no longer has any influence because the flywheel counter continues to operate and supplies pulses at a frequency corresponding to the internal frequency of the flywheel counter. The second output of the flywheel counter supplies a control signal which is applied to the signal combination unit serving as a time slot circuit. Only around the instant when the detection signal from the synchronizing signal detector is expected does the second output of the flywheel counter supply a control signal such that the signal combination unit passes the detection signal and the flywheel counter can be started again. Thus, if the synchronizing signal detector were to supply a detection signal falling outside the aforementioned time slot (for example, because the blocks of information accidentally comprise information which is equal to the synchronizing signal - false lock -), that is to say, if a false detection has taken place, the flywheel counter is prevented from being started again.
However, the device has the drawback that it does not always function satisfactorily and that the synchronization is completely lost, for example, because all pulses in the detection signal supplied by the synchronizing signal detector fall outside the time slot. It is an object of the invention to obviate this drawback and to provide a device which can capture the synchronization (very) rapidly and supplies a synchronizing signal in the rhythm of the synchronizing signal in the electric signal.